Air flow measurement systems incorporating any of various types of sensor elements are well known, for example, a heated wire whose temperature is an inverse function of the velocity of air flowing past the wire. In most air flow measurement devices, the sensor interacts with only an extremely small portion of the total flow field, typically central to the field. It is a well known problem in the design and use of air flow measurement devices that changes in upstream ducting configuration, such as manipulation of an upstream throttle valve, or using a given device in a differently-configured duct or manifold, can alter the flow field around the sensor and thus alter the sensor output non-linearly, i.e., the output signal will contain a degree of error. Therefore, it is known in the art of air flow measurement to provide an air flow conditioning device ahead of the sensor to reduce the degree of error in the sensor output over a desired range of flow measurements.
Many types of air flow conditioning devices exist. Straight sections of pipe tend to allow the flow perturbations to equalize and subside before reaching the sensor. Although simple in concept, the allotted packaging space may prevent the installation of a pipe section of sufficient length; typically, a straight pipe having a length of at least ten diameters should precede the sensor. Tube bundles and honeycomb-shaped bundles can effectively eliminate cross-flow conditions, but they require a means of locating and securing the conditioning device in the upstream duct, and they introduce additional restrictions to flow through greatly increased surface area. These considerations also apply to screens, fins, grids, and various porous media intended for eliminating cross-flow conditions.
Another technique to reduce the sensitivity to upstream flow field variations is to place the sensor at the throat of a nozzle, or venturi. Since the entrance to the venturi covers a larger cross-sectional area of the flow field than does the throat, the venturi effectively samples more of the flow field. It also accelerates the flow of gathered air past the sensor, thereby increasing the gain of the sensor.
As air moves over the surface of the venturi, a boundary layer forms on the walls of the venturi in which the air velocity is zero at the venturi surface and is at the velocity of the main flow at the top of the boundary layer. Beginning at the leading edge of the solid surface, the boundary layer increases in thickness in the direction of air flow. As the boundary layer grows, it acts to reduce the throat area of the venturi, which further accelerates the flow past the sensor.
As much flexibility as possible in the mounting and positioning of air flow measurement systems is preferred. Devices with fewer requirements as to upstream ducting configurations, while still providing the desired measurement accuracy, will have a significant advantage over competing devices having less latitude. Having fewer restrictions as to the upstream ducting configuration means that the flow field presented to the air flow measurement device can exhibit greater variation.
In prior art venturi nozzles, when the flow approaches the surface at a severe off-axis angle such as that caused by a sharp bend in the upstream ducting, the thickness of the boundary layer along the nozzle wall can change abruptly. Such changes result from the transition from laminar to turbulent flow, or even separation of the boundary layer from the surface if the angle of attack is great enough. The flow separates from the nozzle wall at different locations, depending upon the shape and location of the upstream ducting. The thus-modified boundary layer changes the effective area of the nozzle throat, causing the sensor to produce a signal value different than if the same bulk flow rate had a straight-on approach to the venturi.
The location of the separation point depends upon many factors, including flow rate, angle of attack of the air onto the wall, and laminar or turbulent flow. All of these conditions impact the effective throat area of the venturi; hence, the velocity over the sensor will vary with changing upstream conditions even if the actual mass flow rate remains constant, thus generating an erroneous signal.
In the patent literature, US Patent Publication No. US 2001/0037678 discloses a conditioning device having an inlet-side bypass passage containing a venturi-like throttle unit leading to the sensor, whereby the sensor is purported to be isolated from turbulence in the main air flow field. A potential shortcoming of this device is the assumption that flow through the bypass is a constant percentage of the total flow at all flow rates.
US Patent Application Publication No. US 2004/0055570 discloses a conditioning device which essentially divides the air flow approaching a sensor into concentrically central and outer regions, and diverts the outer regions around the sensor, on the assumption that the central region is more laminar and that variations in upstream duct configurations will result in variations in turbulence principally in the outer regions which bypass the sensor.
U.S. Pat. No. 6,267,006 discloses an upstream flow conditioner comprising a conical section leading to a mass flow sensor and having radial fin-like flow conditioning elements (FCEs) which purportedly shape the airflow pattern presented to the mass flow sensor to provide an airflow of uniform velocity with a low magnitude of turbulence fluctuations.
What is needed in the art is an improved air flow measurement device which exhibits a reduced sensitivity to changes in the upstream air flow field.
It is a principal object of the present invention to provide an air flow measurement device which exhibits reduced sensitivity to upstream variations in air flow turbulence.